What if I don't change my Master Cylinder on a disc conversion?

AJ/FormS said
And to equium;
-Almost right.The residual valves are pressed into the exhaust ports of the M/C.I have never pulled one out. I have never converted a drum/drum M/C.
-IIRC that brass valve doubles as the sealing surface for the specially formed M/C brake line. That would mean, once you get it out, you would have to defeat it, then reinstall it so that the joint wont leak.
-Can anyone confirm that?

I misspoke with my earlier statement. I was confusing the residual valve for the Brake Warning switch. Let's try this again:

MC front port --->Brake Warning Switch ---> R and L Calipers
MC Rear port ---->aftermarket Prop valve ---> Brake Warning Switch ---> rear brakes

Since my car was originally Drum/Drum, I only have the Brake warning switch, not a combination/prop valve.